Because they are so physically thick, accretion discs in AGNs are also optically thick. Accretion disks have similar temperature structures. Except at the very shortest wavelengths, the luminosity of an accretion disc at a given wavelength depends on its surface area and hence on the square of the effective radius. This readily permits one to calculate what the size of the region emitting a given wavelength will be. Continuum reverberation mapping of 14 AGNs by [1]} indicated that the sizes of AGN accretion discs were larger than predicted. Using the very different technique of microlensing, [2]}, [3]} found that the inferred sizes of the accretion discs in 10 AGNs were also too large. This is referred to as the “accretion disc size problem". If it is a real discrepancy, it means that something is seriously wrong with our standard optically-thick accretion disc model for AGNs.
